Known devices of the aforesaid type use a spool of special dimensions, different from those of the spools usually used for the external welding of pipes. This has resulted in numerous problems up to the present time, mainly the fact that two different spool types have to be constantly available, resulting in supply and stocking problems, spools for internal welding being moreover not always easily available as they are of special type. These spools are also of relatively high cost compared with the spools usually used for external welding.
Replacing the spool is also relatively lengthy and complicated.
As a matter of fact, in the known devices to change a spool first the welding units have to be removed from their welding position to a replacement one and then the spool and, generally also the attached brake, may be removed.
It is to be noted that to obtain a good welding a correct position of the welding units is essential and therefore after a change of a spool the position of said units has to be checked and adjusted.
Checking or adjusting the welding wire feed means is not easy in the known devices because also for this operation welding units have to be removed from their welding position in order to perform said controls, and this causes the drawbacks mentioned before.
Moreover in most devices the configuration of the joints of the brake coaxial to the spool exposes it to frequent damage because to remove the spool the brake must generally also be removed.
U.S. Pat. No. 3,551,636 describes a device for internally butt-welding pipes wherein each of the welding units comprises a spool on which welding wire is wound and which is mounted with its axle fitted into support notches in the rear ends of a wire supply trailer frame. The trailer frame is pivotally attached to a movable drive frame of the device. A brake may be applied to the spool. In this case, when a spool is empty and has to be removed from its support notches, also the brake is removed. Therefore, it is necessary to use special spools incorporating a brake or else detach the brake from the empty spool and mount it on a new one.
In known devices of the aforesaid type the welding wire feed means also have considerable drawbacks.
This means comprise a motor of particularly sophisticated and costly type, together with two reduction gears, one epicyclic and the other angular.
Although the known feed means are highly efficient they have proved difficult to assemble and maintain, and are also of extremely high cost.